Subcutaneous injections are frequently used in the medical treatment. Retractable safety syringe injectors are especially suitable for subcutaneous injection, which has the following advantages:                1. The retractable safety syringe cylinder is safe and retractable so as not to hurt anybody.        2. The retractable safety syringe cylinder has a self-destroy function. If an overlarge force is used to break the syringe cylinder, the needle will not be ejected out.        3. It is disposable after used once.        4. The use of the retractable safety syringe cylinder is similar to the conventional injector, it can be operated by a simple training.        
FIG. 12 shows the structure of a retractable safety syringe cylinder. It contains a syringe cylinder 1, a syringe needle 2, a push rod 3, and others. The upper end of the syringe cylinder 1 has a syringe connecting seat 11 having a positioning hole 110 therein. The hole has a positioning groove 111 for retaining the syringe needle 2. The push rod 3 has a rod cap 30 which has a hook body 301. The needle of the syringe needle 2 has a slot 201. A periphery of the needle has a ring retained by the positioning groove 111, as shown in FIG. 11.
The use of the retractable safety syringe cylinder will be described herein. At first, the injector is checked. After injection, the hook body 301 can be inserted to the slot 201 correctly. The syringe needle 2 is exactly combined to the hook body 301 at a top of the push rod 3. When the push rod 3 is pulled backwards, the syringe needle 2 will retract into the syringe cylinder 1. When the push rod 3 is pressed again, the syringe needle 2 is bent and deformed in the syringe cylinder 1 so as to prevent from hurting anybody as it is deserted.
Although this retractable safety syringe cylinder has many advantages, when the push rod 3 is pulled backwards, the push rod 3 will rotate and thus the hook body 301 can not be inserted into the slot 201. Thus, the push rod 3 must be rotate again so that the hook body 301 and the hook groove 201 are engaged again. The operation is inconvenient.